Biogas refers to a gaseous fuel produced by the biological breakdown of organic matter in the absence of oxygen. It is produced by the anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material and crops. Biogas primarily comprises methane and carbon dioxide, and may contain small amounts of hydrogen sulfide, moisture and siloxanes.
The gases methane, hydrogen, carbon monoxide, and any other hydrocarbon (e.g., ethane, propane, butane, etc.) can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel. Biogas can be used as a fuel for any heating purpose. It can also be produced by anaerobic digesters where it is typically used in a gas engine to convert the chemical energy of the gas into electricity and heat. Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen, also used for industrial or domestic purposes to manage waste and/or to release energy.
The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. These bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.
Anaerobic digesters can use a multitude of feed stocks for the production of methane rich bio-gas including but not limited to purpose-grown energy crops such as maize. Landfills also produce methane rich bio-gas through the anaerobic digestion process. As part of an integrated waste management system, this bio-gas may be collected and processed for beneficial use while simultaneously reducing greenhouse gas emissions into the atmosphere.
Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas that can be used directly as cooking fuel, in combined heat and power gas engines or upgraded to natural gas quality biomethane. The utilization of biogas as a fuel helps to replace fossil fuels. The nutrient-rich digestate and/or Leachate that is also produced can be used as fertilizer.
H2S is a common contaminant in biogas applications such as landfills and digesters. Previously, biological H2S systems have been used for the removal of H2S from such biogas applications. In some cases, biological H2S removal systems can be several orders in magnitude lower in cost than expensive sulfur removal systems such as media or iron chelating systems. However, conventional biological systems can require more than 2% oxygen to maintain a stable removal rate of H2S, and most raw gas from landfills and digesters contains far less than 2% oxygen. Accordingly, the raw gas from landfills and digesters cannot be processed using a conventional biological H2S removal system due to the scarcity of oxygen.
In biogas applications such as landfills and digesters, H2S and other impurities including halides and halogenated compounds are frequently present in low percent to ppm/ppb quantities. These compounds may dissociate at high temperatures and in the presence of water to form caustic acids including, but not limited to H2S, HF, H2SO4, H3PO4 and HNO3. Typical metallurgy such as carbon and stainless steels are susceptible to corrosion and failure when placed into contact with these acids. Downstream equipment that changes the dew point and allows condensation to occur may concentrate these acids in pooling areas such as moisture separators, chillers, and gas coolers.
Gas processing techniques and other unit operations may produce acids form gas constituents. However, these systems merely employ acid neutralization after the acids have formed and concentrated in the pooling areas. As such, these conventional systems simply act as a band aid to condensation. Caustic scrubbers have been used in the past for several applications. For example, they may be used for CO2 removal, H2S removal and also for the removal of several other reactive contaminants in both liquid and gaseous phase.
The technical expertise required to maintain industrial scale anaerobic digesters coupled with high capital costs and low process efficiencies have limited the level of its industrial application as a waste treatment technology. As a result, it is imperative that anaerobic digesters and biogas treatment plants operate at the highest possible efficiency.